The diagnosis of deafness at an early stage is most important to enable the early fitting of hearing aids and in the application of educational programs to assist language development in the hearing impaired child. Current procedures in the early diagnosis of deafness include the "Cribogram" and brainstem evoked response.
Auditory evoked potentials recorded from the scalp in humans have now been described in many studies. These potentials have been classified into three main groups. These groups are:
(i) brainstem evoked potentials which are approximately 0.5 microvolts in amplitude and occur during the first 10 milliseconds following the presentation of an abrupt sound stimulus, usually a click. PA1 (ii) the middle latency responses which are approximately two microvolts in amplitude and occur between 7 and 50 milliseconds following the presentation of a click or tone pip and, PA1 (iii) slow responses, about 10 microvolts in amplitude, following the onset of a tone burst and have latencies between 50 and 500 milliseconds.
Currently, the brainstem potential is receiving most attention both as a neurological and an audiological tool. It does, however, have the disadvantage of using abrupt stimuli. This is necessary since this response reflects synchronous firing patterns in the auditory pathway in the brainstem. Stimuli of slower onset fail to achieve the synchrony necessary for the recording of the various peaks. As a result of this limitation only high frequency hearing information is measured.
The middle latency responses are also currently receiving attention as a measure of low frequency hearing with low frequency tone bursts being repeated forty times per second to evoke a periodic response. This response is affected by sleep and therefore has limited application in the testing of babies.
The periodic 40 Hz middle latency responses are a subgroup of the auditory steady-state evoked potentials. These are periodic responses, recorded from the scalp to a continuous periodically varying stimulus, for example an amplitude modulated tone. The periodicity of the response is the same as the period of the modulation waveform. These can be recorded over a wide range of modulation and carrier frequencies.
Several classes of subject can be tested by evoked potentials only (or most conveniently) during sleep, for example infants and young children and people with mental retardation.
Existing steady-state literature shows that for low modulation rates (less than or equal to 60 Hz) responses are variable in sleeping subjects and considerably smaller in amplitude than for the waking state.
Responses at all modulation frequencies do decrease with sleep or sedation but we have found that the background noise level cf the EEG decreases dramatically at high modulation frequencies and consistent responses remain present. (see FIG. 11 and FIG. 12). This means that detection of responses in sleep can easily be performed at high modulation frequencies (greater than 60 Hz).